This may be something to include in the substitution support of the WYSIWYG editor now, but…
--- on iOS isn’t really possible — iOS substitutes a double hyphen for a dash, so you can never get to three hyphens in a row. To get <hr>s, I’ve been typing
- - - and then going back and carefully removing the spaces.
Linear solves this by using
___ (triple underscore) instead.
You may have a more capable solution for this (like a menubar with buttons) in the WYSIWYG, but flagging this here nonetheless.